Zheng, playing at home for the first time since winning Paris gold, swatted aside 71st-ranked Russian Kamilla Rakhimova 6-1, 6-1 in 75 minutes.
Sabalenka in contrast laboured in the first set before defeating tricky Thai qualifier Mananchaya Sawangkaew 6-4, 6-1 in her first match since winning the US Open.
Sabalenka and Zheng are on course to meet in the semi-finals in a repeat of the Australian Open final, where the Belarusian crushed her Chinese opponent 6-3, 6-2, before beating her convincingly again in the US Open quarter-finals.
But with the crowd screaming her on, Zheng should be a different proposition in the Chinese capital.
The fifth seed plays Nadia Podoroska of Argentina.
"The atmosphere was insane. I never thought it would be so full in the centre court," said Zheng, whose victory march was momentarily held up by rain.
"When I was on court today I saw that there weren't any empty seats. I was a little bit shocked.
"I haven't seen this kind of thing for a while."
The world number two Sabalenka made hard work in the first set of an opponent ranked 187 in the world and playing the biggest match of her young career.
Sabalenka barely raised a smile at the end after eventually easing to a straight-sets victory.
"I'm super-happy to get through this match," the 26-year-old said.
"In the first set, I was struggling a little bit with the rhythm, with the speed of the court, with the way balls are getting heavier.
"In the second set seems like everything started working much better."
The 22-year-old Mananchaya may have been inexperienced at this level but the athletic Thai refused to give up seemingly lost causes.
Mananchaya struggled on serve, and three-time major champion Sabalenka broke for a 4-2 first-set lead.
The Thai refused to be overwhelmed by Sabalenka's far greater power and she broke back immediately, to roars of approval from the Beijing crowd.
An error-prone Sabalenka could not get into her groove but created a first set point on the Thai's serve, only to put a straightforward volley wide.
She made no mistake on her second opportunity and then broke for 3-1 in the second set as she belatedly found form.
Sabalenka next faces 68th-ranked Ashlyn Krueger of the United States.
Also into the third round of the WTA 1000 event is third seed Jasmine Paolini, but the Italian did it the hard way, fighting back to beat Denmark's Clara Tauson 1-6, 7-5, 6-4.
World number one and defending champion Iga Swiatek pulled out before the tournament because of "personal matters."
